When the Missouri River spilled over its banks in a catastrophic 2011 flood, we could have seen it coming — from space, that is. There's more to the story than meets the eye: the satellites don't take photos of snowpacks or rivers, but rather, they detect tiny changes in gravity over the Earth's surface to track water.
The pair of satellites in question make up NASA's GRACE mission, originally designed to monitoring the melting of polar ice sheets. Together, they orbit 137 miles apart above the Earth, measuring the precise distance between each other and to Earth. If you remember from high school physics, gravity is proportional to mass, so a local buildup of, say, ice or water on the Earth's surface would perturb the satellites' orbits. Thus GRACE measures the Earth's gravitational field and, by extension, the movement of water on the Earth's surface.
"Just like a bucket can only hold so much water, the same concept applies to river basins," J.T. Reager, lead author on a new study about predicting floods, told LiveScience. By identifying saturated river basins, GRACE could help scientists predict areas most vulnerable to flooding. When Reager and his team analysed the data leading up the 2011 Missouri River floods, they could see it coming 5 to 11 months in advance. Field observations of soil and snow, in contrast, could only predict a month or two out.
Reager et al. Nature Geoscience
There are, of course, some limitations to this model. Reager's study only analysed the data retrospectively, and we'll have to see how well it holds up in future floods. It also can't predict flash floods from monsoons. But NASA is working to make data from GRACE available for scientists more quickly, so that we could one day have more time to prepare for these catastrophic floods. [Nature Geoscience via LiveScience]
Picture: Flood water from the Missouri River in Corning, MO in 2011. AP Photo/Dave Weaver